Fix edge case with registered color computation.
The issue is that we track the 1em as a custom reference and thus fail
to compute the color. But in this case 1em is not a valid value and we
should fall back to the initial value.
Differential Revision: https://phabricator.services.mozilla.com/D216030
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1905604
gecko-commit: d33a0841dc39b2de3ee177de880bd643b32f6c24
gecko-reviewers: firefox-style-system-reviewers, devtools-reviewers, nchevobbe, zrhoffman